InvisibleHand is a Firefox extension which can notify if the product you’re currently browsing available somewhere else at a cheaper price. It searches competing merchant sites for the same product and when available displays a discreet notification showing a better deal.

Apart from showing the best price among retailers, it also displays the percentage and actual money saved. The application currently supports more than 50 of US and UK retailers including Wal-Mart, Dixons, Amazon, Best Buy, Newegg, PC World etc.,